Antennas for receiving energy produced by electromagnetic radiation are well known. Conventional antennas are primarily detectors for sensing the electric field component of the electromagnetic radiation. Typically, in conventional antennas the electric field component of the electromagnetic radiation induces a voltage in the antenna when resonant. A conventional antenna is referred to as "electrically small" if its size is less than one-quarter of the wavelength of the electromagnetic radiation to be received. The utility of these antennas is directly related to the wavelength of the electromagnetic radiation, the size of the antenna, and other known loss factors of the antenna. Attempts to construct an efficient, electrically small, antenna have met with several obstacles.
Generally, the size of conventional antennas is tuned to about one-quarter of the wavelength of the electromagnetic radiation to be received. Typically these antennas (i.e., dipoles) have bandwidths of less than 20% of their resonant frequency for useful operation. Larger bandwidths can be obtained with the so-called "frequency independent" antennas (i.e., equiangular spirals), however, even they tend to have maximum bandwidths of about 10:1 (i.e., 2-18 GHz). Here the bandwidth is set by the size of the antenna being .lambda./3 at the lower end of the band and by the electrical size of the antenna feed on the high end of the band. In either case, the size of conventional antennas places at least a lower limit on the frequency of electromagnetic radiation that can efficiently be received. Also, the size of an efficient, low frequency antenna can be prohibitive for most platforms, consequently, efficiency is often sacrificed to make them smaller. For example, a .phi..sub.0 dBi antenna for detecting 1 MHz signals would have to be 400 feet in diameter.
Another obstacle to constructing an electrically small conventional antenna is that a reduction in the size of the antenna generally results in a corresponding reduction in its bandwidth. This is due to the sensitivity of a conventional antenna being a strong function of the wavelength of the received electromagnetic radiation. In other words, the bandwidth is determined by the fact that electrically small antennas must be resonant in order to effectively and efficiently absorb power from the incident energy. Since electrically small antennas also have a small impedance as seen at the antenna feed, this means that additional methods for achieving resonance will be narrow band.
Still another obstacle associated with conventional antenna systems is the limited linear dynamic range of any preamplifiers connected to the passive antenna. Typically semiconductor preamplifiers have about a 100 dB linear dynamic range in the power output of the amplified signals over a 1 Hz bandwidth. In many applications this dynamic range, along with the associated sideband level increase (due to nonlinearities), is unacceptable. Quite often linear dynamic range requirements of over 130 dB may be required in a 1 Hz bandwidth.
Further, since the efficiency of conventional antennas is reduced with their size, noise and other inherent losses become more important when post-processing the induced signal. The increased reduction in efficiency for small antennas is an unavoidable consequence of the low radiation resistance compared to resistive losses of the antenna. Still further inefficiency for small antennas can result from any impedance mismatch between the antenna impedance and the feed line impedance which is typically 50 ohms.
For these reasons, it has been difficult, if not impossible, to produce an electrically small, wide bandwidth, high sensitivity antenna with an amplifier that has a large dynamic range.
These results are further exacerbated when supergain or superdirectivity principles are applied to conventional small antennas. Superdirectivity refers to the ability of an electrically small antenna to have the same antenna pattern as an electrically larger antenna. Superdirectivity is typically obtained by producing a phased array of closely spaced conventional antennas. For traditional phased arrays the spacing of the elements is typically less than one half wavelength at the highest operating frequency. Consequently, the size of the antenna element will determine the phased array bandwidth. For superdirective arrays with even smaller inter-element spacing the size of each antenna becomes even more important. Furthermore, all of the previously described inefficiencies exist along with further reductions in the antenna efficiency due to the strong mutual coupling between the plurality of closely spaced antenna elements. Consequently, superdirectivity arrays are inefficient and impractical when constructed with conventional antenna technology.
Still other attempts to provide an electrically small, high bandwidth antenna have employed superconducting quantum interference devices (SQUIDs) as the preamplifier for an antenna. Nancy K. Welker et al., "A Superconductive H-Field Antenna System," Laboratory for Physical Sciences, College Park, Md. ("the Welker article"). FIG. 2 of the Welker article provides a schematic illustration of the manner in which the SQUID preamplifier is coupled to the antenna in an attempt to improve the bandwidth and sensitivity of an electrically small antenna. Although the proponents of the system maintain that they improve the bandwidth and sensitivity of small antennas, this approach nonetheless suffers from some of the same disadvantages of the conventional antenna systems discussed above. For example, the pickup loop used is inherently narrow band due to its size and method of construction (i.e., the use of resistors and capacitors). Furthermore, they used a single inefficient RF biased SQUID which in part resulted in a much larger pickup loop and reduced linear dynamic range.
Accordingly, it is desirable to provide a small antenna capable of wideband operation. It is further desirable to provide a small antenna capable of wideband operation that is efficient and has a large linear dynamic range. It is further desirable to provide an antenna that is not limited to detecting the electric component of electromagnetic radiation but rather produces an output signal in response to the magnetic field component.